The discovery of novel and useful materials depends largely on the capacity to make and characterize new compositions of matter. As a result, recent research relating to novel materials having useful biological, chemical, and/or physical properties has focused on the development and implementation of new methods and systems for synthesizing and evaluating potentially useful chemical compounds. In particular, high-speed combinatorial methods have been developed to address the general need in the art for systematic, efficient, and economical material synthesis techniques as well as for methods to analyze and to screen novel materials for useful properties.
Generally, it is important to control the quality of the starting materials in any chemical synthesis process. Otherwise, the integrity of the process and the quality of the resulting product would be compromised. Quality control of the starting materials is a particularly important issue in combinatorial synthesis procedures. In such procedures, for example, those employed in peptide drug discovery applications, a large number of starting compounds may be dispensed in a predetermined sequence from a compound library to synthesize a batch of a drug containing a specific peptide sequence. Should any of the starting compounds contain an unacceptable level of a contaminant or exhibit an unacceptable degree of degradation, the resulting compound may be rendered useless. In effect, all starting compounds employed for the batch synthesis would be wasted. This is particularly problematic when the one or more of the starting compounds are rare or expensive.
Similarly, combinatorial testing techniques may be employed in analytical and testing procedures. For example, a plurality of pharmacologically active candidate compounds may be delivered to a test sample in combination in order to assess whether synergistic effects are achieved. If any one of the candidate compounds is compromised in quality, however, the accuracy and reliability of the assessment may be reduced. Thus, further testing may be necessary, adding significantly to the overall time and cost associated with the combinatorial testing process.
High-speed combinatorial methods often involve the use of array technologies that require accurate dispensing of fluids each having a precisely known chemical composition, concentration, stoichiometry, ratio of reagents, and/or volume. Such array technologies may be employed to carry out various synthetic processes and evaluations. Array technologies may employ large numbers of different fluids to form a plurality of reservoirs that, when arranged appropriately, create combinatorial libraries. In order to carry out combinatorial techniques, a number of fluid dispensing techniques have been explored, such as pin spotting, pipetting, inkjet printing, and acoustic ejection. Many of these techniques possess inherent drawbacks that must be addressed, however, before the fluid dispensing accuracy required for the combinatorial methods can be achieved. For instance, a number of fluid dispensing systems are constructed using networks of tubing or other fluid-transporting vessels. Tubing, in particular, can entrap air bubbles, and nozzles may become clogged by lodged particulates. As a result, system failure may occur and cause spurious results. Furthermore, cross-contamination between the reservoirs of compound libraries may occur due to inadequate flushing of tubing and pipette tips between fluid transfer events. Cross-contamination can easily lead to inaccurate and misleading results.
Acoustic ejection provides a number of advantages over other fluid-dispensing technologies. In contrast to inkjet devices, nozzleless fluid ejection devices are not subject to clogging and its associated disadvantages, e.g., misdirected fluid or improperly sized droplets. Furthermore, acoustic technology does not require the use of tubing or involve invasive mechanical actions, for example, those associated with the introduction of a pipette tip into a reservoir of fluid.
Acoustic ejection has been described in a number of patents. For example, U.S. Pat. No. 4,308,547 to Lovelady et al. describes a liquid drop emitter that utilizes acoustic principles to eject droplets from a body of liquid onto a moving document to result in the formation of characters or bar codes thereon. A nozzleless inkjet printing apparatus is used in which controlled drops of ink are propelled by an acoustic force produced by a curved transducer at or below the surface of the ink. Similarly, U.S. patent application Ser. No. 09/964,212 describes a device for acoustically ejecting a plurality of fluid droplets toward discrete sites on a substrate surface for deposition thereon. The device includes an acoustic radiation generator that may be used to eject fluid droplets from a reservoir, as well as to produce an acoustic wave for detection that is transmitted to the fluid surface of the reservoir to become a reflected acoustic wave. Characteristics of the reflected acoustic radiation may then be analyzed in order to assess the spatial relationship between the acoustic radiation generator and the fluid surface. Thus, acoustic ejection may provide an added advantage in that the proper use of acoustic radiation provides feedback relating to the process of acoustic ejection itself.
Regardless of the dispensing technique used, however, inventory and materials handling limitations generally dictate the capacity of combinatorial methods to synthesize and analyze increasing numbers of sample materials. For instance, during the formatting and dispensing processes, microtiter plates that contain a plurality of fluids in individual wells may be thawed, and the contents of selected wells then extracted for use in a combinatorial method. When a pipetting system is employed during extraction, a minimum loading volume may be required for the system to function properly. Similarly, other fluid dispensing systems may also require a certain minimum reservoir volume to function properly. Thus, for any fluid dispensing system, it is important to monitor the reservoir contents to ensure that at least a minimum amount of fluid is provided. Such content monitoring generally serves to describe the overall performance of a fluid dispensing system, as well as to maintain the integrity of the combinatorial methods.
In addition, during combinatorial synthesis or analysis processes, environmental effects may play a role in altering the reservoir contents. For example, dimethylsulfoxide (DMSO) is a common organic solvent employed to dissolve or suspend compounds commonly found in drug libraries. DMSO is highly hygroscopic and tends to absorb any ambient water with which it comes into contact. The absorption of water dilutes the concentration of the compounds in the DMSO as well as altering the ability of the DMSO to suspend the compounds. Furthermore, the absorption of water may promote the decomposition of water-sensitive compounds.
A number of patents describe the use of acoustic energy to assess the contents of a container. U.S. Pat. No. 5,507,178 to Dam, for example, describes a sensor for determining the presence of a liquid and for identifying the type of liquid in a container. The ultrasonic sensor determines the presence of the liquid through an ultrasonic “liquid presence sensing means” and identifies the type of liquid through a “liquid type identification means” that includes a pair of electrodes and an electrical pulse generating means. This device suffers from the disadvantage that the sensor must be placed in contact with the liquid.
U.S. Pat. No. 5,880,364 to Dam, on the other hand, describes a non-contact ultrasonic system for measuring the volume of liquid in a plurality of containers. An ultrasonic sensor is disposed opposite the top of the containers. A narrow beam of ultrasonic radiation is transmitted from the sensor to the open top of an opposing container to be reflected from the air-liquid interface of the container back to the sensor. By using the round trip transit time of the radiation and the dimensions of the containers being measured, the volume of liquid in the container can be calculated. This device cannot be used to assess the contents of sealed containers. In addition, the device lacks precision because air is a poor conductor of acoustic energy. Thus, while this device may provide rough estimate of the volume of liquid in relatively large containers, it is unsuitable for use in providing a detailed assessment of the contents of reservoirs typically used with combinatorial techniques. In particular, this device cannot determine the position of the bottom of containers since substantially all of the emitted acoustic energy is reflected from the liquid surface and does not penetrate to the bottom. Small volume reservoirs such as microtiter plates are regular arrays of fluid containers. The location of the bottoms of the containers in such arrays can vary by a significant fraction of the nominal height of a container due to bow in the plate. Thus, detection of the position of the liquid surface only leads to significant errors in height and thus volume estimation in common containers.
Thus, there is a need in the art for improved methods and devices that are capable of monitoring the contents of a plurality of reservoirs, a capability that is particularly useful in synthetic and analytical processes to increase the robustness, efficiency, and effectiveness of the combinatorial techniques employed therein.